1. Field of the Invention
The present invention relates to a steel wire rod or bar (hereafter occasionally abbreviated to steel) having good cold deformability and to machine parts made therefrom. More particularly, the present invention relates to a steel wire rod or bar having excellent cold deformability without the need for heat treatment to soften after hot rolling. The present invention is especially suitable for making machine parts, such as bolts and nuts, by cold deforming processes, such as cold forging, cold heading, and cold roll forging.
2. Description of the Related Art
Cold deforming is widely used to produce bolts and nuts and other machine parts. Because of its higher productivity and hence higher yields, cold deforming is more efficient than hot deforming and machining. The steel wire rod or bar used for cold deforming should have low flow stress and high workability. High flow stress will reduce the life of tools used for cold deforming; low workability will be subject to cracking during cold deforming, which leads to defective products.
It has been common practice to carry out various heat treatments to soften steel wire rods and bars, such as spheroidizing annealing and annealing prior to cold deforming in order to lower the flow stress and increase the workability of the steel. The annealing step makes the steel wire rod or bar soft and workable enough for cold deforming. Unfortunately, spheroidizing annealing takes a long time (10-20 hours), and, from the standpoint of productivity improvement, energy saving, and cost reduction, there has been an earnest demand for the development of a steel wire rod or bar which exhibits good cold deformability without requiring spheroidizing annealing.
There have been proposed several methods of producing a steel wire rod or bar which exhibits good cold deformability without heat treatment to soften. In these methods, attention is paid to solute C and solute N as in the case of the present invention. They are exemplified below.
(1) Japanese Patent Publication No. 35249/1986 discloses a method of suppressing work hardening due to strain aging, thereby reducing flow stress, as the result of controlling the rolling and cooling conditions, thereby reducing the content of solute C and solute N.
(2) Japanese Patent Laid-open No. 158841/1981 discloses a method of producing a hot-rolled wire rod that is suitable for long die life by employing Ti or B as an element to form nitrides.
(3) Japanese Patent Laid-open No. 39002/1992 discloses a method of producing a hot-rolled wire rod that is suitable for long die life by controlling the Al/N ratio. These two methods are based on the finding that hardness and work hardening are reduced if solute N is fixed.
(4) Japanese Patent Laid-open No. 63635/1982 discloses a method of producing a steel wire rod f or cold forging, which permits an extended tool life, by keeping the steel for 5 hours or more at a temperature between the A.sub.cl transformation point and the A.sub.cl transformation point minus 50.degree. C., thereby solidifying cementite sufficiently and fixing solute N through the controlled Al content.
The above-mentioned four methods are designed to fix solute C and solute N which adversely affect the reduction of flow stress. However, the objects of the above-mentioned references are achieved by controlling the chemical composition of the steel or controlling the rolling and cooling conditions. Nothing is found in the above-mentioned disclosures about the present inventors' discovery that when more than a prescribed number of nitride and carbide particles precipitate in the ferrite particles, the content of solute N and solute C is very effectively reduced and the flow stress is suppressed not only in the initial stage of cold deforming but also in the working stage where the temperature is nearly 100 to 350.degree. C. In fact, the disclosures (2) to (4) above mention nothing about the reduction of flow stress in the later stage of working. No one has ever studied the relation between the flow stress and the number of nitride and carbide particles in the ferrite structure, and the present inventors are the first to study it.